Enterocytozoon bieneusi is the most prevalent microsporidian species in AIDS patients but a tissue culture system is lacking and few animal models exist for this microsporidian. Mansfield and colleagues at the New England Regional Primate Research Center (Amer. J. Pathol. 1997;150:1395-1405) recently reported on monkeys that were spontaneously infected with E. bieneusi, and Tzipori et al. (J. Infect. Dis. 97:1016-1020, 1997) reported the successful experimental transmission of E. bieneusi to monkeys. We therefore repeated an attempt to infect 10 SIV-infected rhesus macaques with E. bieneusi from the stool and duodenal lavage aspirate from human AIDS patients. Spore shedding was detected in stools collected one week later and has continued for approximately four months. Lesions associated with microsporidiosis, as well as parasites were identified in the gall bladder, liver, and small intestine of two monkeys that have been necropsied. Additional studies are underway to establish long-term culture of E. bieneusi and to infect small laboratory animals. To date, attempts to culture E. bieneusi have been unsuccessful. Inoculation of E. bieneusi into dexamethasone-suppressed gerbils has resulted in transient shedding of low numbers of microsporidia